The phytohemagglutinin isolectins E4 and L4 inhibit the growth and proliferation of cultured L1210 murine leukemia cells; however, no evidence of cell death has been noted. The plant glycoproteins have cytostatic properties and demonstrate that the reversible binding of a molecule to the surface of a malignant cell can modulate synthetic pathways and the rate of cell proliferation. Humans undergo a diurnal variation in their peripheral blood colony forming units yielding a maximum at 9:00 a.m.